1. Field of the Invention
This invention relates to an impact type printer.
2. Description of the Prior Arts
Recently, there have been introduced the digital technique and the servo-control technique into the field of character printing control. As the result of this, for example, speed and position controls of a motor to drive a character type wheel having a various character type faces are widely performed by detecting its rotational position. In order to carry out such speed and position controls of the motor with high precision, it is necessary that a circular disc provided on the motor be provided with as many radial slits as possible around its circumference. Also, in recent years, the printing apparatus tends to be miniaturized by the use of a photo-encoder. As the result of this, there is used a small-sized disc having a multitude of radial slits in it. In case such a disc is used, a space interval between the slit disc and a detector largely governs the characteristic of the photo-encoder. Also, one of the important factors to determine the number of the slits to be formed resides in the light source for the photo-encoder. Therefore, unless the light source capable of generating parallel light beams of high intensity is used, the intended size-reduction in the slit disc and increase in the number of the slits therein are impossible. With a view to attaining this purpose, there has so far been used a light source having a high intensity such as, for example, an incandescent lamp, an infrared ray lamp, LED, and so forth, with which parallel light beams are produced by an optical system using a lens, etc. Accordingly, such light source device is inevitably large in size in comparison with the slit disc and surrounding apparatuses therefor. In particular, when the photo-encoder is incorporated in a carriage, there accrues such disadvantage that not only the driving source, such as a motor to drive the carriage, becomes large, but also the apparatus per se, incorporated with the carriage, becomes inevitably large in size.
On the other hand, there have been developed various kinds of type wheels to be provided in the printer, in which each of the types is fixedly provided at the tip end parts of each a group of of radial spokes. Of these type wheels, the one in which the types are integrally formed with wheel made of a plastic material, or the one in which the types made of a plastic material are incorporated in the spokes of different material are advantageous in that they are cheap in the manufacturing cost, while they have such a serious disadvantage that their service life is short. The reason for such disadvantage is that those types bearing symbols, e.g., ".sub.-- ", ",", ".", ".about.", etc., which are usually located off the center of the striking hammer, are liable to hit the platen askew at the time of printing to cause the edge of the type to wear, or that, in the case of the type with metal plating applied on the plastic base, they bring about exfoliation of such metal-plated layer, as the result of which the entire type wheel becomes necessary to be replaced due to damage caused to only one part of the type bearing such symbol.
Also, a flying type impact printer, which is operated by rotating a type wheel provided with matrix types at a high speed, energizing a printing hammer in synchronism with positioning of a selected type on the wheel, when it arrives at a predetermined position, to impart striking force to the chareacter type, thereby performing the printing, has such a disadvantage that it causes digression of the type from its right printing position, because the printing is performed in the state of the type wheel being in rotation.
Further, another impact type printer, which is operated by rotating the type wheel or an endless type belt, on which a plurality of types are held through resilient supporting piece, to select a printing position of the type to be printed, and performing the character printing by striking the type by a printing hammer to the platen over the top surface of an ink ribbon and printing paper, also has such a diadvantage that considerable noise occurs, due to impact at the time of the printing.
Incidentally, the impact type printer can be broadly classified into a type drum printer and a type wheel priner. Various other kinds may also be contemplated, besides these kinds. The type wheel printer uses the type wheel having a plurality of types provided at the tip end parts of a plurality of radially extending spokes. This type wheel is controlled for its lateral movement in parallel with, and along, the axial line of the platen. When a type as selected from the type wheel arrives at a predetermined printing position, the rear surface of the type is struck by a printing hammer against resilient force of the spokes to thereby perform the character printing.
Usually, arrangement of the types on the type wheel is such that each and every type is arranged on one and the same diameter of the wheel with an equal area. However, the area to be occupied by each and every type differs from character to character, i.e., some occupy the whole area, while the others occupy only a small portion of the ara. For the type occupying the small portion of the area, there may be enumerated a point (.), a comma (,), an underline (.sub.--), and small letters of the alphabet.
Therefore, when the impact force of the printing hammer to each of the types on the type wheel is so adjusted as to be adapted to print the character of a large area portion such as, for example, capital letters of an alphabet, the impact force of the printing hammer to the types having smaller character area as mentioned above becomes excessive, since the impact force of the printing hammer concentrates on the small character area, as the result of which the printing force to the platen (i.e., the impact force per unit area of the type) becomes more excessive than in the case of the type having large character area. As the result of this, the printing density of the small type becomes higher than that of the large type, and the density as a whole becomes non-uniform and the resulted print appears poor in quantity. This is a common disadvantage in principle both for the abovementioned type wheel printer and the impact type printer of the other system.
Furthermore, there has been known a printer, in which a selected type is struck by a printing hammer at its position for printing to thereby effect printing. Such apparatus is provided with a device for selecting one type out of a train of types, but it has inevitably an extremely complicated mechanical construction.
There has also been known an impact type printer equipped with a type belt on a carriage movable in parallel with the platen, in which the type belt is rotated by a motor to select an desired type on the type belt at its printing position, and the thus selected type is then struck by a printing hammer to perform the printing operation. Such impact type printing apparatus is required to have such a construction that a type bearing member may be readily mounted or dismounted, or replaced, when it becomes necessary to do so. Another advantage with this type of the printer is that, in most cases, the hands and clothing of operates tend to become stained.